Peripheral Component Interconnect (PCI) is a local computer bus for attaching hardware devices in a computer and is part of the PCI Local Bus standard. The PCI bus supports the functions found on a processor bus but in a standardized format that is independent of any particular processor's native bus. PCI eXtensions for Instrumentation (PXI) is one of several modular electronic instrumentation platforms in current use. Other platforms include PXI Express, CompactPCI, and CompactPCI Express, for example. These platforms are used as a basis for building electronic test equipment, automation systems, modular laboratory instruments in science, and the like. PXI is based on industry-standard computer buses and permits flexibility in building equipment. Often modules are fitted with custom software to manage the system.
PXI is designed for measurement and automation applications that need high-performance and a rugged industrial form-factor. With PXI, modules can be selected from various vendors and be integrated into a single PXI system. A full width PXI chassis may contain up to 18 PXI slots.
PXI uses PCI-based technology and an industry standard governed by the PXI Systems Alliance (PXISA) to ensure standards compliance and system interoperability. There are PXI modules available for almost every test, measurement, and automation application, from the switching modules to high-performance microwave vector signal generation and analysis. PXI is based on CompactPCI. PXI adds a rugged CompactPCI mechanical form-factor, an industry consortium that defines hardware, electrical, software, power and cooling requirements. Then PXI adds integrated timing and synchronization that is used to route synchronization clocks, and triggers internally.
Most PXI instrument modules are register-based products, which use software drivers hosted on a PC to configure them as useful instruments, using the power of PCs to improve hardware access and simplify embedded software in the modules. The open architecture allows hardware to be reconfigured to provide new facilities and features that are difficult to emulate in comparable bench instruments.
PXI modules providing the instrument functions are plugged into a PXI chassis which may include its own controller running an industry standard operating system such as Windows XP, Windows 2000, or Linux (which is not yet PXI System Alliance approved), or a PCI-to-PXI bridge that provides a high-speed link to a desktop PC controller. Likewise, multiple PXI racks can be linked together with PCI bridge cards, to build very large systems such as multiple source microwave signal generator test stands for complex ATE applications.
PXI Express (PXIe) is an adaptation of PCI Express to the PXI form factor. PXI Express also allows for the use of hybrid slots, compatible with both PXI and PXIe modules.
Test and measurement instrumentation has been moving from traditional box instruments to a more modular format like PXIe, which provides several advantages to product designers and customers, for example a smaller footprint, more flexible configuration, and ease of upgrade. While traditional box instruments have always been thought to provide higher measurement performance that modular instruments simply cannot provide, this notion has changed with recent advancements in technologies which have enabled designers to implement high performance systems in the modular format like PXIe and AXIe, for example Keysight's family of VSAs (Vector Signal Analyzer) and VSGs (Vector Signal generator) (e.g. M9393A, M9381A).
Due to the limited PCB space available on a PXIe module, the high performance RF instruments are typically broken up into separate functional blocks which are implemented on multiple modules. For example, the Keysight M9393A VSA instrument includes four modules, a reference clock, synthesizer, downconverter, and an IF digitizer module. A VSA or VSG instrument within a chassis needs a reference clock module, for example, to generate a low phase noise 100 MHz reference clock for all instruments in the chassis. This reference clock is connected to the instrument modules using coaxial cables, such as SMA (SubMiniature version A) cables. A drawback of this approach is the need for the reference clock module which takes up a slot in the chassis.
The PXIe chassis natively provides a 100 MHz reference clock to all slots via the backplane. However, the quality of this clock may not be high enough for high performance RF applications like the VSA. Being a modular instrument, where these modules can be installed in a PXIe chassis from any vendor, there is currently no way to guarantee that the native PXIe chassis reference clock is good enough for the instrument.
In view of various shortcomings of conventional approaches, there may be a general need for a modular electronic instrumentation platform chassis with an isolated reference clock unit, that does not take up a slot in the chassis, and that provides a clean reference clock for all instruments in the chassis.